<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        .father {
            width: 300px;
            height: 500px;
            border: 1px solid red;
        }
        .son1, .son2 {
            width: 100px;
            height: 100px;
            background-color: pink;
        }
        .son1 {
            margin-bottom: 50px;
        }
        .over {
            overflow: hidden;
        }
        .son2 {
            background-color: purple;
            margin-top: 100px;
        }
    </style>
</head>
<body>
<div class="father">
    <div class="over"> 
        <!-- over隔开，那么 son1 就和son1 不是同一BFC -->
        <div class="son1"></div>
    </div>
    <div class="son2"></div>
</div>
</body>
</html>